What we may think of as the old, positivist pursuit of diachronic sound change was, in the last third of the nineteenth century, the new new thing (recall that it was from the Neogrammarians that Saussure emerged).

Then the Neogrammarians patched this theory by adding reasons for reinforcing the deviation such as simplification of sounds, or children imperfectly learning the speech of their parents.

Undoubtedly, the most influential school of historical linguistics is that of the Neogrammarians, one of the main spokesmen being Hermann Paul.